1. Field of the Invention
The present invention relates to a measurement apparatus utilizing electrodes to measure physical properties of an aqueous fibrous solution, and particularly to a technique of measuring physical properties of wetstock in a sheetmaking machine.
2. State of the Art
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (wet stock) on a traveling mesh papermaking fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is essentially a de-watering system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendering rollers at the end of the process. Papermaking devices well known in the art are described, for example, in xe2x80x9cHandbook for Pulp and Paper Technologistsxe2x80x9d 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and xe2x80x9cPulp and Paper Manufacturexe2x80x9d Vol. III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
U.S. patent application Ser. No. 08/766,864, now U.S Pat. No. 5,891,306, describes a sensor that measures water weight on the wire of a paper machine. The sensor detects changes in resistance of the wetstock between the electrodes in an electrode array. The resistance of the wetstock between the electrodes is dependent on the amount of water above the electrodes (i.e., the water weight) and the conductivity of the water. Since the conductivity of the water changes from time to time, the resistance measurement does not uniquely determine the amount of water unless some correction for the conductivity is provided. Consequently, the sensor also includes a separate reference cell which is designed to cancel out all affects that change the resistance between the electrodes other than the water weight. For instance, the resistance measurement is affected by changes in conductivity due to changes in the wetstock temperature or chemical composition. The reference cell electrode configuration is designed to have the same configuration as the measurement cell electrode configuration such that they have the same sensitivity to these conductivity changes. In particular, the spacing between electrodes is the same for both the reference cell and the measurement cell. The reference cell is positioned in a container (such as a bucket) outside of the sheetmaking machine having a continuous flow of white water provided from the sheetmaking machine. The white water of a sheetmaking machine is the water that is drained from the wire which is subsequently recycled. The depth of the white water on top of the reference cell in the container is fixed. Because the depth of water above the reference cell is fixed, any resistive changes detected by the reference cell are due to conductivity changes caused by properties other than water weight (i.e., chemical or temperature). Since the reference cell and measurement cells have the same sensitivity to conductivity, the changes in resistance due to changes in conductivity of the reference cell can be converted into a feedback signal which adjusts/compensates the input test signal Vin coupled to the electrode measurement array so that all resistance changes detected by the measurement cells are due to changes in water weight and not in conductivity changes due to chemical or temperature changes.
There are two main problems with this technique. First, the reference sensors within the container become dirty very quickly and give erroneous readings and hence do not provide a feedback signal that correctly compensates the Vin signal. Consequently, the measurement taken by the measurement array can provide erroneous readings. Moreover, the conductivity of the recycled water may be different than the water on the wire being measured. For example, fiber in the wetstock on the wire carries an ionic charge which may cause the water on the wire to be different than the recycled water with little or no fiber. Hence, the compensation or feedback signal provided by the reference cell may not provide an accurate compensation signal.
The present invention is based in part on the development of a sensor apparatus for measuring electrical characteristics of an aqueous fibrous composition. The apparatus comprises an electrode configuration that is sensitive to at least the following properties of the composition: the conductivity (or resistance), the dielectric constant, and the proximity of the material (e.g., fibrous composition) to the electrode configuration and also comprises a reference electrode configuration. The electrode configuration and measurement apparatus of the present invention includes measurement and reference electrode configurations which allow the sensor apparatus to determine a first property of the aqueous fibrous composition by obtaining resistive measurements corresponding to the first property as well as a second property from both the reference and measurement electrodes.
In one aspect, the invention is directed to a measurement apparatus including at least one measurement electrode cell and a corresponding reference electrode cell. The measurement electrode cell and reference electrode cell have a given sensitivity to a first property of the aqueous fibrous composition and a given sensitivity to a second property of the aqueous fibrous composition. The measurement apparatus obtains simultaneous measurements from both measurement and reference cells. Each of the measurement electrode cell and reference electrode cell have an associated measurement response function to the two properties wherein the resistance (R) measured by each electrode cell is related to the first property (P1) and the second property (P2) as follows: R=f(P1 and P2). Measurement and reference response function equations can be solved using the simultaneously obtained resistance measurements and using previously determined characterization data to determine the first property.
In one embodiment, the response function of each of the measurement and reference electrode cells to the two properties is multiplicative (e.g., R=f1(P1)xc3x97f2(P2). The measurement electrode cell and reference electrode cell have a different sensitivity to the first property of the aqueous fibrous composition and the same sensitivity to the second property of the aqueous fibrous composition. In this case, the ratio of the simultaneous measurements obtained from the measurement electrode cell to the reference electrode cell cancels out the affects from the second property. The determined measurement ratio is used to obtain a measurement of the first property of the aqueous fibrous composition by using previously determined characterization data of the first property vs. a range of measurement ratios. In one embodiment, the first property is the water weight of the aqueous composition and the second property is the specific conductivity of the composition. In this case, the measured resistance R, is R=xcfx81xc3x97f(ww)(where xcfx81=specific resistivity (xcexa9 cm)=1/specific conductivity and f(ww) is a function of the water weight).
In another embodiment, the response of each of the measurement and reference electrode cell to the first and second properties is linearly additive such that R=(Axc3x97P1)+(Bxc3x97P2) (where A and B are constants and P1 and P2 are measures of the two properties). In this embodiment, the measurement electrode cell and the reference electrode cell are designed to have a different sensitivity to both the first and second properties such that the measurement apparatus is characterized by two linear equations:
Rmeasured =AP1+BP2
Rreference =CP1+DP2
where Rmeasured and Rreference are the measured responses of the two sensors, P1 and P2 are the unknown measures of the two properties, and A, B, C, and D are calibration constants. In order to determine the first property, the two equations are simultaneously solved for P1 and P2 using standard linear algebra techniques and using previously determined characterization data to provide calibration constants A-D.
In one embodiment, the characterization data is obtained off-line. At least one measurement electrode cell is used to take measurements of a sample aqueous fibrous composition so as to obtain characterization data of a range of resistance vs. water depth of the measurement electrode cell. In one embodiment, the off-line measurement electrode cell is designed to have the same sensitivities to the first and second properties of the material as an on-line measurement electrode cell. At least one off-line reference electrode cell having the same sensitivities to the first and second properties as the on-line reference electrode cell is used to take measurements of the sample composition so as to obtain characterization data of a range of resistance vs. water depth of the off-line reference electrode cell. In the case in which the two properties have a multiplicative relationship, the ratio of the two sets of characterization data vs. water weight provides a range of resistive ratios vs. water weight characterization information that is sensitive to water depth but not sensitive to changes in conductivity of the water. The resistive ratio vs. water weight characterization data is then used during on-line measurements. To perform an on-line water weight measurement, simultaneous on-line resistive measurements are obtained from each of the on-line reference and on-line measurement electrode arrays. Next, the water weight is determined by using the determined ratio of the measured resistances and the resistive ratio vs. water weight characterization data.
In one embodiment, a plurality of reference electrode cells are built into the on-line measurement electrode array. The measurement electrode array includes corresponding measurement cells and reference cells. Each cell includes an electrode of which a measurement signal Vin is applied through an impedance element. Each cell further includes a corresponding grounded electrode portion. The separation or spacing between the electrode coupled to the resistive element and the grounded electrode portion determines the sensitivity of the electrode cells to the water depth.
In accordance with the embodiment in which the response of each of the measurement and reference electrode cells to the two properties is multiplicative, the spacing of the measurement electrode cell is different than the spacing of the reference electrode cell such that the measurement and reference electrodes have a different sensitivity to water depth but have the same sensitivity to a second property (e.g., conductivity). In one embodiment, the reference cell has a closer spacing so as to have less of a sensitivity to water weight. In one embodiment, the reference cell electrodes are round. In other embodiments, the reference cell electrodes have other shapes which allow the electrode separation to be different from that of the measurement electrodes.
In one embodiment, the electrode configuration and measurement apparatus of the present invention is used in a sheetmaking system such that the built-in reference cell is effective in correcting for changes in conductivity of wet stock (which are not a result of water weight changes) detected by a measurement cell at the wire of a sheetmaking system. In this embodiment, the ratio of the reference cell resistance to the measurement cell resistance is sensitive to water weight changes (first property) while being insensitive to conductivity (second property) which are not a result of water weight changes.